The efficiency of an electric machine depends on the losses in the machine. Losses occur as Joule losses or iron losses for example. The losses are released as heat.
In large permanent magnet direct drive generators these losses come from copper losses in stator conductors. The losses result in high temperature in the coils and especially in the winding overhang that is surrounded by air.
The stator winding temperature must be kept low to avoid decreasing the insulation lifetime, so an efficient cooling system is of prime importance.
It is knows from the prior art to cool the winding overhang and the air gap with air guided through the winding overhang and the air gap.
It is also known to cool electric machines with a liquid. Liquid cooling is a cheap way to increase the cooling capacity, but leakages may lead to electrical problems in the machine. Especially the cooling in wind turbine generators must be reliable and compact.
It is known from the prior art to cool the lamination of an electric machine by guiding a cooling liquid within a duct through the lamination packages of the stator.